The work described in this thesis uses the fission yeast Schizosaccharomyces pombe as model system to analyse the molecular processes that control passage through the G1 and S-phases of the cell-cycle. In particular, the work analyses the control of expression of cdc22+, a gene whose transcript varies in abundance during the cell cycle with a maximum at the G1-S phase boundary. Chapter two describes the sequencing of cdc22+ and shows it encodes the large subunit of ribonucleotide reductase, an enzyme required for DNA precursor metabolism. In the next Chapter, cis-acting elements, resembling MCBs previously identified in budding yeast, are shown to be present 5' to the cdc22+ open reading frame. MCBs can confer cell cycle expression on a heterologous gene in fission yeast, implicating them in controlling periodic expression of cdc22+. A trans-acting complex that specifically binds MCBs is identified, and called DSP1-for b DNA b synthesis control in S.b pombe. DSP1is related to DSC1, an MCB binding activity identified in budding yeast. Experiments in Chapter 3 demonstrate that the gene product of the cdc10+ START gene, p87cdc10, isa component of DSP1. Furthermore, cdc22+ is shown to be constitutively over-expressed in a cdc10 mutant, cdc10-C4. Over-expression of cdc22+ in cdc10-C4 is recessive to wild-type. It is proposed that DSP1 containing p87cdc10-C4 is hyperactive and deregulated as a transcription complex. In summary, p87cdc10 is part of a transcription complex that controls expression of cdc22+, agene required for DNA synthesis. Thus this work demonstrates a molecular link between START and S-phase.